1. Field of the Invention
The present invention relates to a transmitter for processing an electric signal which is based on a process variable and outputting the result of the signal processing, as well as to a method for testing the transmitter. Particularly, the invention relates to a two wire process control transmitter which deals with pressure, temperature, flow rate, and the like, as well as to a method for testing the transmitter.
2. Description of the Prior Art
Basic functions of conventional transmitters are detecting a process variable and transmitting the detected process variable. In addition, some conventional transmitters are used for detecting a malfunction (see Patent Literature 1, for example), and others are used for temporarily changing a 4–20 mA standard range output into an abnormal value (see Patent Literature 2, for example).
One conventional transmitter will hereinafter be described with reference to FIG. 1. FIG. 1 is a block diagram showing the conventional transmitter.
The embodiment in FIG. 1 will here be explained. FIG. 1 is an embodiment of a two wire process control transmitter, where a transmitter 5 is connected to a power unit (distributor) 1 and to a load 3 via a transmission line 2. Normally, a current of 4–20 mA is output from the power unit 1 and flows through the transmission line 2, the transmitter 5, and the load 3, all connected in series.
The transmitter 5 is provided with an built-in display meter (LCD) 6. A communication terminal 7 is connected to the transmission line 2 and provided with a display unit 8 and a keyboard 9.
Further, the transmitter 5 detects a process variable such as static pressure, pressure differential, temperature, and flow rate by the use of sensors (not shown), further, the transmitter 5 converts the detected process variable into an electric signal, and processing the signal by the use of a microprocessor (not shown) to output 4–20 mA based on the electric signal to the transmission line 2.
The process variable becomes the 4–20 mA standard range output current and is applied to the load 3. In this way, the conventional example of FIG. 1 transmits the process variable information.
A detection processing means 200′ included in the transmitter 5 will hereinafter be described with reference to FIG. 2. FIG. 2 is a block diagram showing the detection processing means 200′ of the conventional transmitter.
The detection processing means 200′ is comprised of hardware and includes a sensor 101 and a microprocessor 102′. The microprocessor 102′ has a firmware processing unit 110′. The microprocessor 102′ is connected to the sensor 101 and a memory (non-volatile storage unit) 103. The firmware processing unit 110′ has an input processing unit 10, a diagnosis processing unit 11, and an output processing unit 12. Information generated by the firmware processing unit 110′ is processed by the microprocessor 102′.
Operation of the conventional example of FIG. 2 will be described below.
Firstly, the steps of the input processing unit 10 are performed. As a result, in the case where the transmitter 5 is comprised of a resonant sensor, for example, pressure/ambient temperature of the process is input as a frequency f, and predetermined signal processing is performed to generate a calculated value A. Thus, the calculated value A is based on the frequency f, and thus is based on the pressure/ambient temperature of the process.
Secondly, steps of the diagnosis processing unit 11 are performed. As a result, if the frequency f is within a predetermined range, the diagnosis processing unit 11 diagnoses that there has not been any failure in the detection processing unit (sensor 101—no failure), whereas if the frequency f is outside the predetermined range, the diagnosis processing unit 11 diagnoses that there has been a failure in the detection processing unit (sensor 101—failure). More specifically, when the frequency f is 0, for example, the diagnosis processing unit 11 diagnoses that the sensor 101 of the detection processing unit is malfunctioning.
Alternatively, if the calculated value A obtained by the signal processing of the frequency f is in a predetermined range, the diagnosis processing unit 11 diagnoses that the process variable is normal. On the other hand, if the calculated value A obtained by the signal processing of the frequency f is outside the predetermined range, the diagnosis processing unit 11 diagnoses that the process variable is abnormal.
Then, the diagnosis information is stored in the memory 103 serving as a storage unit.
Thirdly, the steps of the output processing unit 12 are performed. The output processing unit 12 refers to the memory 103, and where operation is normal, that is, the detection processing unit is not malfunctioning and the process is normal, a current in the range 4–20 mA corresponding to the calculated value A is output. The built-in display meter 6 displays the 4–20 mA standard range output. The display unit 8 of the communication terminal 7 also displays the 4–20 mA standard range output. The conventional example of FIG. 2 transmits the process variable information in the above-described manner.
The memory 103 is checked and when there is a failure of the detection processing unit, the output current falls above or below the 4–20 mA range. As a result, the built-in display meter 6 displays an alarm. Further, the display unit 8 of the communication terminal 7 displays an alarm, too.
In the case where in checking the memory the process is malfunctioning though no failure is detected in the detection processing unit, the value of the 4–20 mA standard range output OUT is kept at the previous value.
[Patent Literature 1] Japanese Patent No. 3308119
[Patent Literature 2] JP-A-2002-175112
However, when conducting an on-the-spot inspection or the like in order to test for a failure in the detection processing unit of the transmitter integrated in a system, it is necessary to partially disable (disassemble) the transmitter in order to confirm behavior of the entire transmitter in the partially disabled state, and manpower and cost are undesirably incurred by such test.
More specifically, in order to temporarily change the values of the built-in display meter, the alarm, and other components in addition to changing the value of the 4–20 mA standard range output to abnormal values, it is necessary to actually disassemble the transmitter deliberately, thereby incurring manpower and money expenditure.